Electrolytic cells comprising a solid sodium ion conductive electrolyte membrane that selectively transports sodium ions are known in the art. By having a sodium ion-selective membrane in the electrolytic cell, sodium ions are allowed to pass between the cell's negative electrode compartment and positive electrode compartment while other chemicals are maintained in their original compartments. Thus, through the use of a sodium ion-specific membrane, an electrolytic cell can be engineered to be more efficient and to produce different chemical reactions than would otherwise occur without the membrane.
Solid sodium ion conductive electrolyte membranes are used in electrochemical cells for various reasons, including, but not limited to, being: ion conductive, ion selective, water impermeable, chemically stable, electronically insulative, and so forth. By way of example, NaSICON (Na Super Ion CONducting) membranes selectively transport sodium cations. Other examples of solid sodium ion conductive electrolyte membranes include beta alumina, sodium-conductive glasses, etc.
Electrolytic cells comprising solid sodium ion conductive membranes are used to produce a variety of different chemicals and to perform various chemical processes. In some cases, however, such cells are used as batteries that can store and release electrical energy for a variety of uses. In order to produce electrical energy, batteries typically convert chemical energy directly into electrical energy. Generally, a single battery includes one or more galvanic cells, wherein each of the cells is made of two half-cells that are electrically isolated except through an external circuit. During discharge, electrochemical reduction occurs at the cell's positive electrode, while electrochemical oxidation occurs at the cell's negative electrode. While the positive electrode and the negative electrode in the cell do not physically touch each other, they are generally chemically connected by at least one (or more) ionically conductive and electrically insulative electrolyte(s), which can either be in a solid or a liquid state, or in combination. When an external circuit, or a load, is connected to a terminal that is connected to the negative electrode and to a terminal that is connected to the positive electrode, the battery drives electrons through the external circuit, while ions migrate through the electrolyte.
Batteries can be classified in a variety of manners. For example, batteries that are completely discharged only once are often referred to as primary batteries or primary cells. In contrast, batteries that can be discharged and recharged more than once are often referred to as secondary batteries or secondary cells. The ability of a cell or battery to be charged and discharged multiple times depends on the Faradaic efficiency of each charge and discharge cycle.
Rechargeable batteries based on sodium can employ a solid primary electrolyte separator, such as a solid sodium ion conductive electrolyte membrane (discussed above). The principal advantage of using a solid sodium ion conductive electrolyte membrane is that the Faradaic efficiency of the resulting cell approaches 100%. Indeed, in many other cell designs, the electrode solutions in the cell are able to intermix over time and, thereby, cause a drop in Faradaic efficiency and loss of battery capacity.
In some cases, the sodium negative electrode in sodium-based rechargeable batteries is molten. In such cases, the batteries may need to be operated at temperatures above about 100° C., the melting point of sodium. Furthermore, some conventional sodium-based batteries comprise a positive electrode change transfer mechanism using a solution (e.g., NaOH) that has a relatively high pH or that is otherwise chemically reactive to the sodium ion conductive electrolyte membrane. As a result of the high operating temperatures and the chemically reactive positive electrolyte solutions, the solid sodium ion conductive electrolyte membrane of some conventional sodium-based batteries is relatively susceptible to degradation by dissolution. Accordingly, the useful life of such batteries may be relatively short.
Thus, while sodium-based rechargeable batteries are known, challenges still exist, including those mentioned above. Accordingly, it would be an improvement in the art to augment or even replace certain conventional sodium-based rechargeable cells with other battery charge transfer mechanisms.